A start up circuit is often required when a power supply circuit supplies voltage to a power circuit within an integrated circuit (IC). The start up circuit provides a starting bias voltage until the power circuit is able to function regularly. Afterwards, the start up circuit is expected to be idle and consume no power, if ideally. FIG. 1 is a diagram illustrating relationships among a start up circuit 10, a power supply 100 and a power circuit 200. During initialization stage, the power circuit 200 has not been provided with power yet. Therefore, it is necessary to provide a start up circuit 10 to charge the capacitor C until the voltage at the node Vbias reaches a predetermined value that is able to turn ON the power circuit 200. After the power circuit 200 is turned ON, it may operate without aid from the start up circuit 10. For example, the power circuit 200 may obtain power from the power supply 100 via some other approach and transfer the power into a low DC voltage Vdd required by the IC. The details are not described here for that they are well known to those skilled in the art.
FIG. 2 is a diagram illustrating a prior art start up circuit 10. Since the start up circuit 10 is expected to consume as little current as possible, the simplest approach to implement the star-up circuit is to provide a resistor 20 of high resistance. The resistor 20 transfers the voltage from the power supply 100 to a low current, charging the capacitor C until the node Vbias reaches a predetermined voltage value. The voltage at the node Vbias, for example, maybe provided to drive a pulse width modulation (PWM) circuit 12 in the power circuit 200, and the power circuit operates under the control of the PWM circuit 12. The details of the PWM circuit and how it controls the power circuit 200 are not described here for that they are well known to those skilled in the art.
According to the prior art illustrated in FIG. 2, the resistance of the resistor 20 must be quite large to limit the current, because the voltage provided by the power supply 100 is quite high. Accordingly, the area of the resistor 20 inevitably becomes very large, and a huge amount of heat is generated. Moreover, such start up circuit cannot be turned OFF; the serious problems of power consumption and heat generation go on even after the power circuit has been started up.
Another start up circuit is disclosed in U.S. Pat. No. 5,285,369 “Switched Mode Power Supply Integrated Circuit with Start up Self Biasing”. The disclosed circuit is very complicated, and a simplified form thereof is illustrated in FIG. 3. This prior art utilizes the characteristics of the parasitic junction transistor inherently existing with a metal-oxide-semiconductor field-effect transistor (MOSFET). As shown in the figure, the MOSFET 84 may be taken as a combination of a junction field-effect transistor (JFET) 86 and a MOSFET 88. The JFET 86 is a depletion mode transistor, inherently capable of limiting current, and it is normally in an ON state as its gate is electrically connected to ground. The node between the JFET 86 and the MOSFET 88 provides current for starting up a control circuit 14. The control circuit 14 provides two functions: on the one hand, the control circuit 14 charges the capacitor C; on the other hand, when a voltage at the node Vbias reaches a predetermined value, the control circuit 14 generates a control signal to switch off the MOSFET 88 and turn off the start up circuit formed by the MOSFET 84 and the control circuit 14.
Though the conventional start up circuit illustrated in FIG. 3 can be automatically turned off and the heat generated by the circuit is much less than that in FIG. 2, the structure of the control circuit 14 is still too complicated(as may be understood by referring to the details thereof), which is undesired.
Therefore, another circuit structure is disclosed in U.S. Pat. No. 5,477,175 “Off-Line Bootstrap Start up Circuit”, which is simpler than the circuit in FIG. 3. As illustrated in FIG. 4, the circuit disclosed in U.S. Pat. No. 5,477,175 obtains current from the node between the depletion mode FET 101 and the MOSFET 102, and transfers the current to voltage by a resistor 103, which is supplied to the gate of the MOSFET 102 to turn ON the MOSFET 102. After the power circuit 200 is started, the transistor switch 109 can be switched ON by controlling the node 113, to pull down the gate voltage of the MOSFET 102. The MOSFET 102 is thus turned OFF.
Though the complexity of the circuit illustrated in FIG. 4 is reduced as compared to the circuit disclosed in U.S. Pat. No. 5,285,369, it is still not satisfactory. First, in order to turn OFF the MOSFET 102, the transistor 109 is kept conductive after the power circuit 200 is started, and thus there is a power consumption path from depletion mode FET 101—resistor 103—transistor switch 109 to ground. Second, the depletion mode FET 101 has a “body effect”. When it supplies current to the resistor 103 to increase the gate voltage of the MOSFET 102, its own source voltage also increases, which deteriorates its body effect and reduces the current flowing through the depletion mode FET 101. In a worst case the depletion mode FET 101 may shut down.
In view of the foregoing, it is desired to provide an improved start up circuit which is free from the drawbacks of power consumption and body effect in the prior art.